The present invention relates to an antenna duplexer used for mobile communication equipment.
In the prior art antenna duplexer, a transmitter surface-acoustic-wave filter is connected with a receiver surface-acoustic-wave filter via a phase shift circuit in the package of the device.
The phase shift circuit can shift the phase of the transmitter-band for the receiver surface-acoustic-wave filter. Furthermore, the phase shift circuit has the phase angle and the characteristic impedance so that the magnitude of the reflection coefficient at a center frequency in the transmitter-band of receiver SAW filter is to be equal to 0.8 or greater and the phase angle of reflection coefficient at a center frequency in the transmitter-band is to be approximately 0xc2x0. The phase shift circuit has the characteristic impedance of about 50 ohms.
Generally, the receiver surface-acoustic-wave filter has, within its pass band, capacitive impedance of less than 50 ohms. Therefore, in the structure above, it is difficult to realize the impedance matching of 50 ohms in both the transmitter-band and the receiver-band, with insertion loss and voltage standing wave ratio (hereinafter referred to as VSWR) increased.
It is the object of the present invention to provide an antenna duplexer having an optimum impedance matching in the transmitter-band and the receiver-band at the antenna port, as well as having improved VSWR characteristics with minimized insertion loss in its pass band.
The antenna duplexer of the present invention addresses the conventional problems, and includes: i) a transmitter surface-acoustic-wave filter; ii) a phase shift circuit whose one port is electrically connected with the output port of the transmitter surface-acoustic-wave filter; and iii) a receiver surface-acoustic-wave filter whose input port is electrically connected with the other port of the phase shift circuit.
The transmitter and receiver surface-acoustic-wave filters have different pass bands in frequency and attenuate the other pass band with each other. The phase shift circuit can at least shift the phase of the receiver surface-acoustic-wave filter in its transmitter-band.
The phase shift circuit can shift the phase of the receiver surface-acoustic-wave filter in its transmitter-band, as well as can control the reflection coefficient at a center frequency in the transmitter-band of the receiver surface-acoustic-wave filter so that the magnitude of the reflection coefficient is to be equal to 0.8 or greater and the phase angle of the reflection coefficient is to be 0xc2x0 through 45xc2x0.
Accordingly this has the same advantage as adding inductive components, in parallel with the antenna port, in the pass band of the transmitter surface-acoustic-wave filter having capacitive impedance of less-than-50 ohms. This allows the impedance in the transmitter-band at the antenna port of the antenna duplexer to match closer to 50 ohms.
According to the present invention, it is thus possible to obtain an antenna duplexer having improved VSWR characteristics and minimize insertion loss in its transmitter-band.
In an embodiment of the present invention, the antenna duplexer contains a transmitter surface-acoustic-wave filter, a phase shift circuit whose one port is electrically connected with the output port of the transmitter surface-acoustic-wave filter, and a receiver surface-acoustic-wave filter whose input port is electrically connected with the other port of the phase shift circuit.
The transmitter and receiver surface-acoustic-wave filters have different pass bands, and attenuate the other pass band with each other. The phase shift circuit can at least shift the phase of the receiver surface-acoustic-wave filter in its transmitter-band.
The phase shift circuit shifts the phase of the receiver surface-acoustic-wave filter in its transmitter-band and has a phase angle and a characteristic impedance so that the magnitude of the reflection coefficient at a center frequency in transmitter-band of receiver surface-acoustic-wave filter is to be equal to 0.8 or greater and the phase angle of the reflection coefficient at a center frequency in transmitter-band of receiver surface-acoustic-wave filter is to be 0xc2x0 through 45xc2x0.
Accordingly, compared with a conventional antenna duplexer, such structured antenna duplexer can easily provide the impedance in the transmitter-band at the antenna port of the device so as to match closer to 50 ohms. That is, the device described above has improved VSWR characteristics and minimized insertion loss in transmitter-band.
In another embodiment of the present invention, in addition to the structure described in the immediately preceding structure, the antenna duplexer includes the phase shift circuit having less-than-50 ohm characteristic impedance.
Since the receiver surface-acoustic-wave filter has, in its pass band, impedance of less than 50 ohms, the phase shift circuit can provide the impedance in the receiver-band at the antenna port of the antenna duplexer to match closer to 50 ohms.
The antenna duplexer has therefore improved VSWR characteristics and minimized insertion loss in the both transmitter-band and receiver-band.
According to still another embodiment of the present invention, in addition to the structure introduced in the immediately preceding structure, the antenna duplexer includes the phase shift circuit whose characteristic impedance takes the range of 42xc2x18 ohmsxe2x80x94not including 50 ohms. The antenna duplexer has therefore improved VSWR characteristics and minimized insertion loss in the both transmitter-band and receiver-band.
According to yet another embodiment of the present invention, in addition to the initially described structure, the antenna duplexer has the transmitter surface-acoustic-wave filter formed of a ladder-type filter, in which the resonance frequency of a serial-branch surface-acoustic-wave (hereinafter referred to as SAW) resonator is higher than the anti-resonance frequency of a parallel-branch SAW resonator.
With the antenna duplexer, broadband filter characteristics can be obtained, with the VSWR characteristics and insertion loss in the transmitter-band maintained at an adequate level.
According to another embodiment of the present invention, in addition to the initially described structure, the antenna duplexer contains the phase shift circuit whose phase angle takes the range of 90xc2x110xc2x0. The antenna duplexer can therefore provide the transmitting band at the antenna port with the impedance matching of 50 ohms, enhancing the VSWR characteristics on the transmitter side, with insertion loss minimized.
According to still another embodiment of the present invention, in addition to the structure introduced in the immediately preceding structure, the antenna duplexer includes the phase shift circuit whose characteristic impedance takes the range of 42xc2x18 ohmsxe2x80x94not including 50 ohms, and whose phase angle takes the range of 90xc2x110xc2x0. This realizes the impedance matching of 50 ohms in the receiver-band at the antenna port of the antenna duplexer. The antenna duplexer has therefore improved the VSWR characteristics and minimized insertion loss in the receiver-band, as well as in the transmitter-band.
According to still another embodiment of the present invention, like the structure described immediately above, the antenna duplexer includes: i) a transmitter SAW filter; ii) a phase shift circuit whose one port is electrically connected with the output port of the transmitter SAW filter; and iii) a receiver SAW filter whose input port is electrically connected with the other port of the phase shift circuit.
The transmitter and receiver SAW filters have different pass bands and attenuate the other pass band with each other. The phase shift circuit can shift the phase of the receiver SAW filter in its transmitter-band. The phase shift circuit has less-than-50 ohm characteristic impedance.
This realizes impedance matching of 50 ohms in the receiver-bandat the antenna port of the antenna duplexer.
The antenna duplexer has therefore improved the VSWR characteristics and minimized insertion loss in the receiver-band.
According to yet another embodiment of the present invention, in addition to the structure introduced in the immediately preceding structure, the antenna duplexer includes the phase shift circuit whose characteristic impedance takes the range of 42xc2x18 ohmsxe2x80x94not including 50 ohms. This realizes impedance matching of 50 ohms in the receiver-band at antenna port of the antenna duplexer. The antenna duplexer has therefore improved the VSWR characteristics and minimized insertion loss in the receiver-band.